This invention relates to a valve, particularly, but not exclusively, for use in connection with a compressor.
Refrigeration and air-conditioning compressors often rely on the pressure difference generated by the compressor to provide the pressure difference needed to inject oil into the compressor for lubricating, sealing and cooling purposes. The pressure at the inlet (or “suction”) of the compressor is dictated by the refrigeration duty being performed at the time. The delivery pressure at the outlet is dictated by the ability of the condenser of the refrigeration system to transfer heat to the condenser cooling medium. This medium may be ambient air or it may be a liquid such as water or a solution containing an anti-freeze. The medium is in turn cooled by ambient air or a cooling tower.
At times, unless preventative action is taken, the pressure difference between delivery and suction can be too low to inject oil into the compressor. This can arise from either a high suction pressure, for example when the plant is started up after being turned off for a period, or it can arise from a low delivery pressure. The latter can again occur after the plant has been turned off for a period, or it can arise where the condenser cooling medium is at a very low temperature. Both high suction pressure and low delivery pressure can be present at the same time. In some cases the condition would clear if the plant were allowed to continue running—but it cannot due to lack of oil injection. In other cases the cooling medium may be so cold that the condition would persist.
To remedy this situation it is necessary to fit means to keep the delivery pressure (as “seen” by the compressor and the oil system) high. One such means is a delivery/suction pressure control valve downstream of the compressor delivery and the oil separator/reservoir but upstream of the condenser. Such a valve is expensive.
Positive displacement compressors that do not have mechanical delivery valves (for example screw and scroll compressors) tend to act as expanders and rotate backwards when the driving force is switched off until the pressures in the evaporator and condenser equalise. This reverse migration of refrigerant from the high to the low-pressure side of the system is undesirable, as is the reverse rotation of the compressor, which can cause mechanical damage. To prevent this a non-return valve (also known as a check valve) is often fitted to the delivery line, downstream of the compressor delivery and the oil separator/reservoir but upstream of the condenser.